1. Field of the Invention
The present invention relates to a visual inspection system of a photomask used for processing semiconductor products and to a visual inspection method thereof.
2. Description of the Related Art
A photomask (referred to as a mask hereinafter) serves as the master mask of a semiconductor integrated circuit pattern, and is used on an optical aligner such as a stepping projection aligner or stepper. The mask is produced in the following processes: a metallic film such as a chromium oxide film having a shade effect is evaporation-deposited over the surface of a glass substrate having a thickness of several millimeters, and the metallic film thereover is subjected to an etching process, to thereby form an integrated circuit pattern thereof on the surface of the substrate. Conventionally, the mask maker has inspected the mask for an opaque defect that the metallic film remains in excess, a clear defect that by contrast, the metallic film pattern forming the integrated circuit pattern is partially broken, and a particle defect that the mask has a foreign particle on its surfaces. The mask maker has rejected the mask having a defect or a particle that is out of size or number specifications.
Furthermore, in order to complement the mask inspection performed by the mask maker, the semiconductor manufacturer who is the mask user has transferred the circuit pattern of the supplied mask to the wafer, and inspected the wafer to determine whether or not the circuit pattern formed on the wafer can be used. Thus, the semiconductor manufacturer makes an effort to increase the yields of their semiconductor products. When by the wafer inspection carried out by the mask user, the defects are detected in the same coordinate position among a plurality of chips, that is, when the common defects among a plurality of chips are detected, there is a high probability that the mask has a deficiency such as a defect. In this case, the mask user finds out a position on the mask in which the defect is located from a coordinate value which represents a position on the wafer in which the common defect detected by the wafer inspection is located, to thereby analyze the causes of the deficiency. At that time, the mask user's wafer inspection conditions have been different from the mask maker's mask inspection conditions. That is, the conditions of setting the direction in which the mask pattern is disposed, the inspection area, the origin of coordinates, and the axes of coordinates, used in the mask inspection previously performed by the mask maker have been different from the ones used in the wafer inspection. Therefore, it has been necessary to transform or apply the data such as the coordinate value or the like which represents the defect area detected by the wafer inspection into the data used in the mask inspection.
FIG. 5 is an explanatory view showing one example illustrating the relation between the pattern located on the wafer and the one located on the mask, and the relation between the origin of coordinates used in the wafer inspection and the origin of coordinates used in the mask inspection. FIG. 5 is also an explanatory view showing one example describing the method of transforming the coordinate value used in the wafer inspection into the one used in the mask inspection. In the figure, “a” designates a pattern located on the wafer, and represented in the coordinate system used in the wafer inspection; “aO” designates the origin of the rectangular coordinates consisting of x-and y-axes included in the wafer inspecting data, which are used for showing the position of the pattern “a” located on the wafer; “b” designates a pattern located on the mask, shown in the coordinate system used in the mask inspection; and “bO” designates the origin of the rectangular coordinate system consisting of x-and y-axes included in the mask inspecting data, which are used for showing the position of the pattern “b” located on the mask. In the figure, the left lower corner position of the pattern “a” located on the wafer is defined by the origin of the coordinate system “aO”, and the left lower corner position of the pattern “b” located on the mask is defined by the origin of the coordinate system “bO”. The pattern “a” located on the wafer and the pattern “b” located on the mask are disposed in the relation in which the pattern “a” is obtained by mirror-reversing the pattern “b” with an optional axis extended in the y-axis direction as a center. Herein, the pattern obtained by scaling down the pattern “b” located on the mask is shown as the pattern “a” located on the wafer.
The inspection and observation of the mask are performed from the side of the mask substrate (glass substrate) on which the pattern (formed in the metal film such as chromium oxide) is located (the pattern-surface side). On the other hand, the transferring (or exposure) of the pattern onto the wafer is performed by irradiating the photoresist formed over the wafer with light from the side of the mask substrate on which the pattern is not located (the glass-surface side). For this reason, the image obtained by mirror-reversing the image located on the pattern-surface side of the mask is formed on the wafer. That is, the image formed on the wafer is the image seen from the glass-surface side of the mask. Therefore, in order to observe a defective area detected by the wafer inspection on the mask, it is necessary to transform a coordinate value representing the position on the wafer into a coordinate value representing a position on the mask. This coordinate transformation converts the coordinate value representing the position of the defect area detected by the wafer inspection into the coordinate value on the mask. The coordinate value thus obtained on the mask is used for the mask observation, and the area in which the defect is located is determined, to thereby analyze the deficiency.
Conventionally, the transformation of the coordinate value used in the wafer inspection into the coordinate value used in the mask inspection has been carried out as mentioned above. However, with a recent increase in the degree of integration of semiconductor products, requirements against defective points and foreign particles located on the mask have been increasingly stricter. Therefore, the mask inspection carried out by the mask maker cannot achieve the sensitivity required for effectively dealing with the defective portion and foreign particle. Moreover, the coordinate value used in the mask inspection does not have direct compatibility with the coordinate value used in the wafer inspection carried out by the mask user so as to complement the mask inspection done by the mask maker. Therefore, it is necessary to transform the coordinate value of the deficient area detected on the wafer in order to observe the mask for analyzing the cause of the deficiency. In addition, the inspection conditions of the mask inspection done by the mask maker vary from one mask maker to another in the position from which the inspection begins and in the direction in which the inspection proceeds, on the circuit pattern formed on the mask. For this reason, the coordinate value data used in the mask inspection and the coordinate value data obtained in the wafer inspection done by the mask user sometimes stand in the relationship in which the former is rotated with respect to the latter with the coordinate values being of opposite sign to each other. It is sometimes necessary to flip the coordinate value 90 or 180 degrees in the coordinate transformation mutually done between the coordinate values. The data showing the results of the mask inspection varies from one mask maker to another in the requirements or specifications. There has been a drawback that it is necessary to transform the coordinate value of the defect area detected by the wafer inspection by use of the transforming method compatible to the inspection performed by each mask maker.
Additionally, because the mask user performs the wafer inspection, and the mask maker performs the mask inspection, there have been the following drawbacks. The data such as the origins of coordinates or the coordinate axes used in each inspection can be misinformed. The deficiency of the mask cannot be analyzed because the coordinate transformation is not properly done due to misunderstanding the inspection data. The analysis of the deficiency takes much time.
Moreover, a wasteful time is taken because miscalculation easily occurs due to the complex transformation of the coordinate value. As a result, there has been a drawback that the production of semiconductor products is delayed because of delayed deliveries of the mask for producing wafers, and thereby serious damage can occur.